The present invention relates to an antenna, in particular but not exclusively, to an antenna for portable radio apparatus.
Antennas for portable radio apparatus are generally required to be small, yet still have good radio frequency propagating characteristics. Conventional rod and spiral wound rod antennas, whilst having good radiation propagation characteristics, will generally be relatively long for example a quarter or three-eighths of a wavelength long. At typical radio telephone frequencies of 900 MHz this would result in a rod antenna, including its feed portion, being in the region of 60-85 mm long. As portable radio apparatus, in particular radio telephones, have reduced in size there has been a corresponding demand for a reduction in the size of the antenna. A typical configuration having a relatively low volume is the helical antenna, and these have been commonly adopted for use with radio telephones. However, such helical antennas are relatively narrow band which makes them unsuitable for radio telephone networks requiring relatively wide band-width operation such as the Japanese Personal Digital Cellular (PDC) radio telephone system which has up and down links centred around 936 MHz and 847 MHz respectively for the 800 MHz frequency band system.